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Model for Moving Bed Coal Gasifier Aspen Plus
Model for Moving Bed Coal Gasifier
Aspen Plus
Copyright (c) 2010-2014 by Aspen Technology, Inc. All rights reserved.
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This software includes NIST Standard Reference Database 103b: NIST Thermodata Engine Version 7.1
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Revision History 1
Revision History
Version Description
V7.2 First version
V7.3 Update the model to V7.3 and add a paragraph in Introduction sectionto describe what files are released.
V7.3.2 Update the model to V7.3.2
V8.2 Update the model to V8.2
V8.4 Update the model to V8.4
V8.6 Update the model to V8.6
2 Contents
Contents
Revision History ......................................................................................................1
Contents..................................................................................................................2
Introduction............................................................................................................3
1 Components .........................................................................................................4
2 Process Description..............................................................................................5
3 Physical Properties...............................................................................................6
4 Reactions .............................................................................................................8
4.1 Coal drying ................................................................................................84.1.1 Drying process ..............................................................................84.1.2 Amount of water vaporized .............................................................8
4.2 Coal pyrolysis.............................................................................................84.2.1 Pyrolysis reaction...........................................................................84.2.2 Amount of each pyrolysis product ....................................................9
4.3 Char gasification and combustion..................................................................94.3.1 Reactions......................................................................................94.3.2 Reaction kinetics............................................................................9
5 Simulation Approach ..........................................................................................12
5.1 Unit Operations ........................................................................................ 135.1.1 Coal drying ................................................................................. 135.1.2 Coal pyrolysis .............................................................................. 135.1.3 Char gasification and combustion................................................... 13
5.2 Streams .................................................................................................. 155.3 Calculator Blocks ...................................................................................... 155.4 Convergence............................................................................................ 15
6 Simulation Results .............................................................................................17
6.1 Parametric analysis................................................................................... 196.1.1 Bed voidage ................................................................................ 196.1.2 Number of RCSTRs in series .......................................................... 20
6.2 Comparison with literature results .............................................................. 21
7 Conclusions ........................................................................................................27
References ............................................................................................................28
Introduction 3
Introduction
This file describes a comprehensive Aspen Plus model for countercurrentmoving bed coal gasifiers.
The model includes the following features:
The model is a steady-state model.
The model considers all the processes occurring in the gasifier, i.e. coaldrying, coal pyrolysis, char gasification, and char combustion.
The kinetics for char gasification and combustion are included.
Coal drying and pyrolysis take place instantaneously at the top of gasifier.
The variable bed voidage throughout the gasifier is taken into account.
The solid and gas phases flow in a plug-flow pattern.
The pressure drop in the gasifier is neglected.
The solid and gas temperatures are equal inside the gasifier.
The following files related to this example can be found in theGUI\Examples\Moving bed coal gasifier folder of the Aspen Plusinstallation:
Aspen_Plus_Model_for_Moving_Bed_Coal_Gasifier.apwz, a compound filecontaining these five files:
o Aspen_Plus_Model_for_Moving_Bed_Coal_Gasifier.bkp
o Aspen_Plus_Model_for_Moving_Bed_Coal_Gasifier.pdf
o USRKIN.f
o USRKIN.dll
o USRKIN.opt
Aspen_Plus_Model_for_Moving_Bed_Coal_Gasifier.bkp
Aspen_Plus_Model_for_Moving_Bed_Coal_Gasifier.pdf
USRKIN.dll
USRKIN.opt
4 1 Components
1 Components
The following table lists the chemical species present in the process:
Table 1. Components Used in the Model
ID Type Name Formula
O2 CONV OXYGEN O2
CO CONV CARBON-MONOXIDE CO
H2 CONV HYDROGEN H2
CO2 CONV CARBON-DIOXIDE CO2
H2O CONV WATER H2O
CH4 CONV METHANE CH4
N2 CONV NITROGEN N2
H2S CONV HYDROGEN-SULFIDE H2S
C6H6* CONV BENZENE C6H6
C SOLID CARBON-GRAPHITE C
S SOLID SULFUR S
COAL NC ------ ------
DRY-COAL* NC ------ ------
CHAR NC ------ ------
ASH NC ------ ------
*: C6H6 represents the tar and DRY-COAL represents the dried coal.
2 Process Description 5
2 Process Description
Moving bed coal gasifiers are vertical countercurrent reactors in which coalreacts with oxygen and steam to produce the gas containing CO, H2, CO2,CH4, and other hydrocarbons. Fig. 1 shows a schematic diagram of a movingbed coal gasifier. Coal is fed to the top of the gasifier and moves downwardunder the gravity. A preheated mixture of oxygen and steam is introduced atthe bottom of the gasifier and flows upward to react with the coal. As coaldescends slowly, four processes will take place in sequence: coal drying, coalpyrolysis, char gasification, and char combustion. Ash and unreacted char areremoved at the bottom by the rotating grate, and the produced gas leaves atthe top. Part of the process steam is produced by a water jacket surroundingthe gasification chamber.
Figure 1. Schematic diagram of moving bed coal gasifier
6 3 Physical Properties
3 Physical Properties
In this model, the property method RK-SOAVE is used to calculate thephysical properties of mixed conventional components and CISOLIDcomponents. HCOALGEN and DCOALIGT models are used to calculate theenthalpy and density of non-conventional components, respectively.
The HCOALGEN model requires these three component attributes for non-conventional components: proximate analysis results (denoted as PROXANALin Aspen Plus), ultimate analysis results (denoted as ULTANAL in Aspen Plus),and sulfur analysis results (denoted as SULFANAL in Aspen Plus). Theproximate analysis gives the weight content of moisture, fixed carbon, volatilematter and ash. The ultimate analysis gives the weight composition of coal interms of ash, carbon, hydrogen, nitrogen, chlorine, sulfur, and oxygen. Thesulfur analysis divides the sulfur content into three types, pyritic, sulfate, andorganic sulfur. The DCOALIGT model requires only the two componentattributes ULTANAL and SULFANAL.
Table 2 shows the component attributes of coal used in our model, which arefrom Wen et al.[1]. The enthalpy and density of coal are calculated by theHCOALGEN and DCOALIGT models, respectively.
For the characterization of the char and ash generated in coal conversion, thesame methodology as above is applied and the same models are used tocalculate their enthalpy and density. The results of proximate, ultimate, andsulfur analyses for the char and ash are determined from the analysis data oforiginal coal and the amount of gaseous product in terms of mass balance.
3 Physical Properties 7
Table 2. Component Attributes of Coal Used in the Model[1]
Proximate analysis Ultimate analysis Sulfur analysis
ElementValue(wt.%)
ElementValue(wt.%, drybasis)
ElementValue(wt.%, drybasis)
Moisture
(wet basis)4.58 C 77.76 Pyritic 0.87
Fixed carbon
(dry basis)39.16 H 5.24 Sulfate 0.87
Volatile matter
(dry basis)52.72 N 1.47 Organic 0.88
Ash
(dry basis)8.12 Cl 0
S 2.62
O 4.79
Ash 8.12
8 4 Reactions
4 Reactions
When the coal travels downward along the gasifier, these reactions take placein sequence: coal drying, coal pyrolysis, char gasification, and charcombustion.
4.1 Coal drying
4.1.1 Drying processIn the coal drying process, the physical moisture bound in the coal is releasedinto the gas phase. The dried coal which results is represented by DRY-COALin the model.
4.1.2 Amount of water vaporizedThe amount of vaporized water is determined based on the water content inthe proximate analysis of coal, because the temperature in the gasifier isusually high enough to vaporize all the bound water in coal. At the same time,from the results of Hobbs et al.[2], we know that the length for coal drying ingasifier is much smaller than that for other processes, such as chargasification and combustion. So, we assume that coal drying takes placeinstantaneously at the top of gasifier.
4.2 Coal pyrolysis
4.2.1 Pyrolysis reactionCoal pyrolysis is to break the coal to form the products of CO, H2, CO2, H2O,H2S, N2, CH4, tar, and char, as shown in Eq. (1). In the model, tar isrepresented by C6H6.
CharHCCHNSHOHCOHCOCoal 66422222 (1)
4 Reactions 9
4.2.2 Amount of each pyrolysis productIn the literature, there are two methods used to obtain the amount of eachcoal pyrolysis product. One is based on experiments, such as a coal pyrolysisexperiment outside the gasifier[3]. The other uses a theoretical method suchas a functional group model[2]. Due to the natural complexity of coal incomposition, the theoretical method is usually very complicated, and isdifficult to use in practical application. Compared with the theoretical method,the method based on experiments is simpler and more practical. So, theexperimental method is used to predict the results of coal pyrolysis in themodel. Additionally, the results of Hobbs et al.[2] show that in the gasifier, thelength for coal pyrolysis is negligible relative to the length for char gasificationand combustion. So, coal pyrolysis is assumed to happen instantaneously atthe top of gasifier in the model.
4.3 Char gasification andcombustion
4.3.1 ReactionsIn the process of char gasification and combustion, reactions (2-7) areconsidered[1, 4]:
221
1
122
2CO
ZCO
Z
ZO
Z
ZC
(2)
22 HCOOHC (3)
COCOC 22 (4)
422 CHHC (5)
222 HCOOHCO (6)
OHOH 222 5.0 (7)
In reaction (2), the parameter
TeCO
COZ
6249
2
2500
, where [CO] and [CO2]
mean concentrations of CO and CO2, respectively; T is temperature in unit ofK[5].
4.3.2 Reaction kineticsReactions (2-5) are the solid-gas reactions. Some of these reactions arevolumetric reactions, while others are surface reactions. In the volumetricreactions, gas can quickly diffuse into the particles and reaction takes placethroughout of the interior of particle. In the surface reaction, gas does notpenetrate into the particle but is confined at the surface of the shrinking core
10 4 Reactions
of unreacted solid. Generally, volumetric reaction occurs when chemicalreaction is slow compared with diffusion. Surface reaction occurs whenchemical reaction is very fast and diffusion is the rate-limiting step. Amongthese four reactions, the rate of reaction (2) is usually fast relative to thediffusion rate of reactants, so reaction (2) occurs as a surface reaction. Therates of the other three reactions are rather slow because of the lowoperating temperature in the moving bed coal gasifier, typically lower than1000ºC. So, reactions (3-5) are volumetric reactions.
Based on the above statements, the unreacted-core shrinking model isapplied to describe the reaction rate of reaction (2)[1]:
dashsfilm
O
OC
kYkk
PR
1112
2
2
(8)
Where
2OCR = reaction rate, mol/cm3·s.
filmk = gas film diffusion coefficient, mol/cm3·atm·s.
sk = chemical reaction constant, mol/cm3·atm·s.
dashk = ash diffusion coefficient, mol/cm3·atm·s.
2OP = partial pressure of oxygen, atm.
particle
core
r
rY , where corer is radius of unreacted core in cm and particler is radius
of feed coal particle in cm.
In the moving bed gasifier, coal particle size is of the order of 1cm[6], and inmost cases, the gas film and ash diffusions are the rate-limiting steps. Then,Eq. (8) is simplified as:
dashfilm
O
OC
kk
PR
112
2
(9)
Where
Td
T
kp
film
75.1
180026.4292.0
.
Y
Ykk pfilmash
1
5.2 .
T = temperature, K.
4 Reactions 11
pd = diameter of coal particle size, cm.
p = porosity of ash, dimensionless. In the model, p = 0.75.
The rates of reactions (3-7) are expressed in Table 3.
Table 3. Reaction Rates of Reactions (3-7)
Reaction Reaction rate Comment Unit Source
(3) *987.1
45000
22930 OHOH
T PPCe
T
COH
OH
e
PPP
1633029.17
* 2
2
mol/cm3·s [1]
(4) *987.1
45000
22930 COCO
T PPCe
T
COCO
e
PP
2028092.20
2*
2
mol/cm3·s [1]
(5) *8078
087.7
22 HHT PPCe
5.0
1010043.13
* 4
2
T
CH
H
e
PP mol/cm3·s [1]
(6)T
P
t
wgs
HCO
OHCO
Tw
ePk
xxxx
eF
t 555391.8
2505.0
987.1
277605
22
2
10877.2
Twgs ek 8.1
72346890.3
mol/s·g ofash
[1]
(7)22
4
315.8
10976.951083.8 OH
T CCe
------ mol/m3·s [7]
Note: T = temperature, K. C = concentration of carbon, mol/cm3. OHP2
,
2COP ,2HP , COP , and
4CHP = partial pressures of components, atm. wF =
correction factor taking into account the relative reactivity of ash to the iron-
base catalyst. In the model, 0084.0wF . tP = total pressure, atm. COx ,
OHx2
,2COx , and
2Hx = mole fractions of components, dimensionless.2HC
and2OC = concentrations of components, mol/m3.
In the Aspen Plus model, the kinetics of these reactions are provided in anexternal Fortran subroutine.
12 5 Simulation Approach
5 Simulation Approach
Fig. 2 shows the simulation diagram of Aspen Plus model. The function ofeach block is described in Table 4. This model covers the processes occurringin the gasifier, i.e. coal drying, coal pyrolysis, char gasification, and charcombustion.
Figure 2. Simulation diagram for moving bed coal gasifier
Table 4. Function of Each Block
Block Model Function
DRYING RYieldSimulate coal drying based on the water content value inproximate analysis of coal
PYROLYS RYieldSimulate coal pyrolysis based on the results of pyrolysisexperiment
GASIF-1···10 RCSTR Simulate char gasification and combustion
CHAR-DEC RStoicDecompose char into C, H2, O2, N2, S, and ash in order toeasily deal with solid reaction in the simulation of chargasification and combustion
5 Simulation Approach 13
Block Model Function
SEP-1···3 Sep2 Separate the gas and solid
MX-GASIN Mixer Mix the gas feedstock
MX-EXCH HeaterMix the product gas and provide the heat for coal dryingand pyrolysis
5.1 Unit Operations
5.1.1 Coal dryingA RYield block, DRYING, is used to simulate the coal drying. The coal is fedinto the block, and the water bound in coal is vaporized in this block. Theyield of gaseous water is determined by the water content in the proximateanalysis of coal. For the coal we are using, the water content is 4.58wt.%, sothe mass yield of gaseous water is set as 4.58%, based on the assumptionthat the physically bound water is vaporized completely in this process. Themass yield of dried coal is correspondingly equal to 1-4.58% = 95.42%.
After the drying process, the gaseous water and dried coal flow into a gas andsolid separator, SEP-1. The separated gaseous water mixes with the gasstreams from coal pyrolysis, char gasification, and char combustion toproduce the final product gas, and the separated dried coal goes on to thenext block for the pyrolysis process. In the simulation diagram, there is aheat stream called Q-DRYING which represents the heat duty in the dryingprocess. This stream is used to keep the heat balance inside the gasifier. Theheat needed in the drying process is provided by the hot gases from coalpyrolysis, char gasification, and char combustion.
5.1.2 Coal pyrolysisThe coal pyrolysis is simulated by a RYield block, PYROLYS. In this block, thedried coal is broken into CO, H2, CO2, H2O, H2S, N2, CH4, C6H6, and char. Theyield of each component is specified according to the results of the pyrolysisexperiment[3]. The heat required in the pyrolysis process originates from theheat exchange with the gas from char gasification and combustion, and isrepresented by the heat stream Q-PYROLYS in the model. After the pyrolysis,the pyrolysis products flow into the block SEP-2 to separate the gas and solid.The gases from SEP-2 flow upward into the coal drying process. The solidchar from SEP-2 flows downward into the char gasification and combustionprocesses.
5.1.3 Char gasification and combustionThe whole gasifier consists of four processes, namely coal drying, coalpyrolysis, char gasification, and char combustion. In the model, coal drying
14 5 Simulation Approach
and pyrolysis are assumed to happen instantaneously at the top of gasifier, asdescribed in sections 4.1.2 and 4.2.2. This indicates that the length for chargasification and combustion is equal to the total length of gasifier in themodel.
The moving bed coal gasifier is a countercurrent reactor. This indicates that acountercurrent reactor model is required to simulate the char gasification andcombustion processes. However, Aspen Plus does not have a built-in reactormodel to deal with the countercurrent reactor. Benjamin et al.[8] developed auser solution program for the countercurrent moving bed coal gasifier, andthen integrated it into Aspen Plus, but their results showed that the solutionwas time consuming. This is attributed to the following reason: The form ofthe mathematical model of the countercurrent moving bed coal gasifier is atwo-point boundary value problem. Its solution requires matching a numberof variables, some specified at the top and others at the bottom of thegasifier. This feature causes the solution process to be usually complicatedand time-consuming. So, from the viewpoint of directly using the built-inalgorithm in Aspen Plus and then simplifying the problem, a number of RCSTRreactors in series are proposed to model the char gasification and combustionprocesses. The RCSTR reactor has the characteristic that all phases have thesame temperature, which means the temperatures of solid and gas phases inthe char gasification and combustion processes are equal in the model.
As suggested above, the simulation for char gasification and combustionprocesses is performed by a series of RCSTRs. However, in order to easilydeal with the solid-gas reactions in this process, a RStoic block, CHAR-DEC, isset up before the series of RCSTRs. In this block, char is decomposed into theelements C, H2, O2, N2, S, and ash. The stoichiometric coefficients of theseelements are determined according to the ultimate analysis of char, which isautomatically done by a Calculator. In the char decomposition, the heat dutyis specified as 0 in the specification sheet of RStoic in order to maintain theheat balance inside the gasifier. The products leaving from CHAR-DEC enter asolid and gas separator, SEP-3. The separated gases including H2, O2, and N2
are introduced into the bottom of the gasifier together with the feedstock O2
and H2O. The separated solid components, including C, S, and ash, go to aseries of RCSTRs to take part in the gasification and combustion reactions.Each RCSTR has the same volume, which is equal to the whole gasifiervolume divided by the number of RCSTR in series. The reaction kineticsdescribed in section 4.3.2 are written in external Fortran code. The heat lossbetween the bed and wall is represented by the heat stream. Each heatstream is bound with a Calculator and its value is determined by thecorresponding Calculator. The Calculator automatically retrieves the reactortemperature in the flowsheet iteration and then updates the value of heatstream based on Eq. (10):
wallreactorloss TTAUQ (10)
Where
lossQ = heat loss, Btu/hr.
U = heat transfer coefficient, Btu/hr·ft2·ºR. In the model, U =
16Btu/hr·ft2·ºR in order to match the carbon conversion with literatureresults[1] in the subsequent simulation.
5 Simulation Approach 15
A = area, ft2.
reactorT and wallT = temperature, ºR.
In Fig. 2, the direction of each heat stream is flowing into the RCSTRs. So,the negative term before the heat transfer coefficient in Eq. (10) is to correctthe direction of heat stream to make it flow out of the RCSTRs. In Fig. 2, 10RCSTRs in series are shown from GASIF-1 to GASIF-10. Ten RCSTRs in seriesare used due to the fact that the simulation results change little as thenumber of RCSTR is further increased.
5.2 StreamsStreams represent the material and energy flows in and out of the process.This model includes two types of streams: material and heat streams, asshown in Fig. 2. The streams with solid lines represent material streams. Thestreams with dashed lines represent heat streams.
5.3 Calculator BlocksThis model includes 11 Calculator blocks, as shown in Table 5.
Table 5. Calculators Used in the Model
Name Function
CHARDECDetermine the stoichiometric coefficients of C, H2, O2, N2, S, and ashin reaction of CHAR-DEC block
QLOS-1···10 Calculate the heat loss for blocks GASIF-1···10
5.4 ConvergenceThe convergence method impacts simulation performance greatly.Inappropriate convergence methods may result in the failure of convergenceor long running time. In this model, the convergence method for RCSTRsfrom GASIF-1 to GASIF-10 blocks is very important. Tables 6 and 7summarize the convergence parameters for each RCSTR block used in theexample model, which are specified on the sheet Blocks | GASIF-1···10 |Convergence | Parameters.
16 5 Simulation Approach
Table 6. Convergence Parameters for Blocks GASIF-1···5
Item ParameterValue
GASIF-1 GASIF-2 GASIF-3 GASIF-4 GASIF-5
Mass balanceconvergence
Solver Broyden Broyden Broyden Broyden Broyden
Maximum iterations 100 500 500 500 100
Energy balanceconvergence
Maximum iterations 100 500 500 500 100
Maximum temperature step 90F 9F 9F 90F 90F
Advancedparameters
Mass balance | Dampingfactor for step size
1 1E-10 1 1 1
Initialization
Initialize using integration |Integration parameters |Corrector | Convergencemethod
Direct Direct Direct Direct Direct
Initialize using integration |Integration parameters |Integration error | Errorscaling method
Dynamic Dynamic Dynamic Dynamic Dynamic
Table 7. Convergence Parameters for Blocks GASIF-6···10
Item ParameterValue
GASIF-6 GASIF-7 GASIF-8 GASIF-9 GASIF-10
Mass balanceconvergence
Solver Broyden Broyden Broyden Broyden Broyden
Maximum iterations 100 100 100 100 500
Energy balanceconvergence
Maximum iterations 100 100 100 100 500
Maximum temperaturestep
90F 90F 9F 9F 9F
Advancedparameters
Mass balance | Dampingfactor for step size
1 1 1 1 1
Initialization
Initialize using integration| Integration parameters |Corrector | Convergencemethod
Direct Direct Direct Direct Direct
Initialize using integration| Integration parameters |Integration error | Errorscaling method
Dynamic Dynamic Dynamic Dynamic Dynamic
6 Simulation Results 17
6 Simulation Results
A Pittsburgh bituminous coal is selected to perform the coal gasificationsimulation. The coal attributes and operational parameters of the gasifier arefrom the open literature of Wen et al.[1], as shown in Tables 2, 8, and 9. Table2 gives the component attributes of coal, specifically the data of proximate,ultimate, and sulfur analyses. Table 8 lists the feed conditions of coal,oxygen, and steam streams, which include the flow rate, temperature, andpressure. In addition, the diameter of coal particles and mole fraction ofimpurity nitrogen are provided in coal and oxygen streams, respectively.Table 9 lists the configuration parameters and operational conditions of thegasifier, including gasifier height, gasifier diameter, operating pressure, andwall temperature. At the same time, the pressure drop along the gasifier isneglected in the simulation. In the paper of Hobbs et al.[2], they consideredthe pressure drop. The nominal operating pressure in the gasifier was100kPa. However, the maximum pressure drop calculated was only about3kPa. This indicates that the pressure throughout the bed changes little andthe pressure drop is negligible. So, for simplification, the pressure drop is notconsidered in the model.
Table 8. Feed Conditions of Feedstocks[1]
Feedstock Parameter Value Unit
Coal
Flow rate 43204 lb/hr
Temperature 77 °F
Pressure 500 psig
Diameter of particle 2.0 cm
Oxygen
Flow rate 25923 lb/hr
Mole fraction of N2 0.06 dimensionless
Temperature 700 °F
Pressure 500 psig
Steam
Flow rate 123132 lb/hr
Temperature 700 °F
Pressure 500 psig
Table 9. Configuration Parameters and OperationalConditions of Gasifier[1]
Parameter Value Unit
18 6 Simulation Results
Parameter Value Unit
Height 7.6 ft
Diameter 12 ft
Pressure 500 psig
Wall temperature 700 °F
In the simulation of the coal pyrolysis process, the yield of each pyrolysisproduct is estimated based on the report of Suuberg et al.[3]. Table 10summarizes the component attributes of coal used in Suuberg’s work and ourmodel. Table 11 shows the yield of each pyrolysis product used in Suuberg’swork and our model. From Table 10, it can be seen that the proximateanalysis of coal used in our model is similar to that in Suuberg’s work.However, there is a big difference in ultimate analysis, especially in thecontent of carbon and oxygen. In Suuberg’s work, the carbon and oxygencontents are 63.63% and 19.53%, respectively. However, in our model, thecarbon and oxygen contents are 77.76% and 4.79%, respectively. From thesedata, it can be inferred that the increase in carbon content may increase theyield of components containing carbon, and that the decrease in oxygencontent may decrease the yield of components containing oxygen. Based onthis assumption, we increase the yield of methane and decrease the yield ofCO, CO2 and H2O compared with the results of Suuberg et al., as shown inTable 11.
Table 10. Comparison of Component Attributes of CoalUsed in Suuberg et al.’s Work[3] and This Model
Proximate analysis (wt.%) Ultimate analysis (dry basis, wt.%)
ElementSuuberget al.
Thiswork[1] Element
Suuberget al.[15]
Thiswork[1]
Moisture
(wet basis)6.8 4.58 C 63.63 77.76
Fixed carbon
(dry basis)39.59 39.16 H 4.08 5.24
Volatile matter
(dry basis)49.79 52.72 N 0.97 1.47
Ash
(dry basis)10.62 8.12 Cl 0 0
S 1.18 2.62
O 19.53 4.79
Ash 10.62 8.12
6 Simulation Results 19
Table 11. Comparison of Yield of Pyrolysis Products inSuuberg et al.’s Work[3] and This Model
ComponentsYield (mass basis on dried coal, %)
Suuberg et al. This model
CO 7.62 1.9 (=7.62/4)
CO2 9.01 2.25 (=9.01/4)
H2O 10.41 0.65 (=10.41/16)
CH4 1.39 13.95 (=1.39×10)
H2 0.54 0.54
tar 5.79 5.79
C2H4 0.6 Not considered
HC 1.02 Not considered
H2S Not measured 0.94
N2 Not measured 0.35
Char 63.62 73.63
Total 100 100
Based on above input conditions, some model parameters, including bedvoidage and number of RCSTRs in series, are analyzed. Then, our simulationresults are compared with literature results[1] to validate the model.
6.1 Parametric analysis
6.1.1 Bed voidageFig. 3 shows the effect of bed voidage on the profile of carbon flow rate. Inthe first case, the bed voidage is kept at a constant value, 0.4, throughoutthe gasifier. In the second case, the bed voidage increases linearly from 0.4at the top to 0.7 at the bottom. In both cases, the number of RCSTRs inseries is 10. From Fig. 3, it can be seen that both the carbon flow rates inthese two cases decrease gradually from top to bottom. But the carbon flowrate decreases slower when the bed voidage increases from top to bottom.This is due to the decrease in carbon consumption rate.
Bed voidage refers to the fraction of the bed not filled with particles, soincreasing bed voidage will cause the particle number per unit volume todecrease. This in turn decreases the carbon consumption rate per unitvolume. The decreasing carbon consumption rate keeps the carbon flow ratehigher in the second case. From these results, it can be seen that accuratesimulation results significantly depend on the correct setup of the profile ofbed voidage in the gasifier.
Some researchers have made attempts to measure the bed voidage in thegasifier. Krishnudu et al.[9] quenched a pilot plant moving-bed coal gasifierand measured bed voidage along the bed length. It was found that the bedvoidage varied linearly from the top to the bottom of the bed. Meanwhile,
20 6 Simulation Results
Hobbs et al.[2] also found that it was necessary for accurate simulation resultsto consider variable bed voidage. Therefore, the second case for the setup ofbed voidage is used in our model, i.e. bed voidage increases linearly from thetop (0.4) to the bottom (0.7).
Figure 3. Effect of bed voidage on profile of carbon flow rate
6.1.2 Number of RCSTRs in seriesFig. 4 shows the effect of number of RCSTRs in series on carbon conversion.The number of RCSTRs in series ranges from 1 to 15. In all cases, the bedvoidage increases linearly from 0.4 (at the top) to 0.7 (at the bottom). As thenumber of RCSTRs in series increases from 1 to 5, the carbon conversionshows a steep increase, which is from 79.5% to 96.5%. However, when thenumber of RCSTRs in series ranges from 5 to 10, the carbon conversionshows a gentle increase, which is from 96.5% to 98.3%. When the number ofRCSTRs in series is further increased from 10 to 15, the increase of carbonconversion becomes much slower, which is from 98.3% to 98.8%. Theseresults indicate that it may be reasonable to use 10 RCSTRs in series for ourcase.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 1 2 3 4 5 6 7 8
Carb
on
flow
rate
(kg/s
)
Distance from bottom (ft)
Linear change of bed voidage
Constant bed voidage
6 Simulation Results 21
Figure 4. Effect of number of RCSTRs in series on carbon conversion
6.2 Comparison with literatureresultsTable 12 shows the simulation results with 10 RCSTRs in series. In thesimulation, the bed voidage increases linearly from 0.4 (at the top) to 0.7 (atthe bottom). For comparison, the results reported by Wen et al.[1] are alsogiven in Table 12. The compared items include the product gas composition,carbon conversion, exit gas temperature and peak temperature in thegasifier. From Table 12, it can be seen that our simulation results show areasonable agreement with Wen’s results.
Table 12. Simulation Results
SourceProduct gas composition (dry basis, mol.%)
Carbonconv. (%)
Exit gastemp. (K)
Peaktemp. (K)
CO H2 CO2 CH4 H2S N2 C6H6
Wen et al.[1] 27.16 38.12 22.84 9.34 0.81 1.70 0.03 98 1063.5 1378.2
This work 28.57 37.70 21.98 9.00 0.28 1.72 0.75 98.3 990.0 1355.1
Figs. 5-8 show the profile of temperature and main components (CO, H2, andCO2). For comparison, the results of Wen et al.[1] are correspondinglysummarized in Figs. 5-8.
75
80
85
90
95
100
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Carb
on
co
nvers
ion
(%)
Number of RCSTRs in series
22 6 Simulation Results
Fig. 5 shows the profile of temperature along with the height of the gasifier.As the distance increases from the bottom to the top, the temperatureincreases quickly to a maximum value and then decreases gradually. Theincrease of temperature is due to the existence of O2. At that time, theexothermic reactions of C-O2 and H2-O2 dominate the change of temperature.When the O2 is consumed, the endothermic reactions of C-H2O and C-CO2
make the temperature decrease.
Fig. 6 is the profile of CO mole fraction. The mole fraction of CO increaseswith the height near the bottom of the gasifier. When a maximum value isreached, the mole fraction of CO begins to decrease in the rest of the height.
Fig. 7 gives the profile of H2 mole fraction. With the increase of height frombottom to top, H2 mole fraction does not increase at first and is kept ataround 0. This is because H2 is consumed up by O2 in the feed gas. When theO2 is consumed, H2 mole fraction shows an increase until the end of thegasifier.
Fig. 8 is the profile of CO2 mole fraction. In the whole gasifier, CO2 molefraction shows a monotonic increase from the bottom to top, except for aslight decrease at the top. Comparing the Aspen Plus model results withWen’s results, it is found that they show a similar trend in the profile oftemperature and main components.
6 Simulation Results 23
(a)
(b)Figure 5. Profile of temperature along with the height of gasifier: (a) AspenPlus model; (b) Wen’s model[1]
0
1
2
3
4
5
6
7
8
100 300 500 700 900 1100 1300 1500
Dis
tan
ce
fro
mb
ott
om
(ft)
Temperature (K)
Gas phase
Solid at the top
Solid at the bottom
24 6 Simulation Results
(a)
(b)Figure 6. Profile of CO along with the height of gasifier: (a) Aspen Plus model;(b) Wen’s model[1]
0
1
2
3
4
5
6
7
8
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
Dis
tan
ce
fro
mb
ott
om
(ft)
Mole fraction
CO
6 Simulation Results 25
(a)
(b)Figure 7. Profile of H2 along with the height of gasifier: (a) Aspen Plus model;(b) Wen’s model[1]
0
1
2
3
4
5
6
7
8
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
Dis
tan
ce
fro
mb
ott
om
(ft)
Mole fraction
H2
26 6 Simulation Results
(a)
(b)Figure 8. Profile of CO2 along with the height of gasifier: (a) Aspen Plusmodel; (b) Wen’s model[1]
0
1
2
3
4
5
6
7
8
0 0.02 0.04 0.06 0.08 0.1
Dis
tan
ce
fro
mb
ott
om
(ft)
Mole fraction
CO2
7 Conclusions 27
7 Conclusions
A comprehensive Aspen Plus model is developed for the countercurrentmoving bed coal gasifier. To provide the model, several Aspen Plus unitoperation blocks are combined. In the model, the kinetics for char gasificationand combustion are considered and provided in an external Fortransubroutine. The model considers variable bed voidage throughout the gasifier.The Aspen Plus model results are in reasonable agreement with the literatureresults[1]. The Aspen Plus model provides a useful modeling framework forfuture refinements as new knowledge is gained about the moving bed coalgasifier.
To use the Aspen Plus model, you need to provide the following data:
Component attributes and higher heat of combustion of coal. Thecomponent attributes of coal include the data of proximate, ultimate, andsulfur analyses.
Feed conditions of coal, oxygen, and steam streams. This includesthe flow rate, temperature, and pressure for coal, oxygen, and steamstreams, and also the diameter of coal particles and mole fraction ofimpurity nitrogen in the oxygen stream.
Configuration parameters and operational conditions of gasifier.This includes gasifier height, gasifier diameter, operating pressure, andwall temperature.
Yield of each coal pyrolysis product from the pyrolysis experiment.
Model parameters. This includes the heat transfer coefficient betweenbed and wall, the porosity of the ash layer, and the reactivity of ash forthe reaction of CO and H2O.
From the model, the following information can be obtained:
Profile of each component flow rate;
Profile of carbon conversion;
Profile of temperature;
Pressure of exit gas and solid;
Gas and solid residence times in gasifier.
28 References
References
[1] C.-Y. Wen, H. Chen, M. Onozaki, “User’s manual for computersimulation and design of the moving bed coal gasifier”, Reportsubmitted to Morgantown Energy Technology Center and U.S.Department of Energy, Contract DOE/MC/16474-1390, 1982.
[2] M.L. Hobbs, P.T. Radulovic, L.D. Smoot, “Modeling fixed-bed coalgasifiers”, AIChE J., 38: 681-702, 1992.
[3] E.M. Suuberg, W.A. Peters, J.B. Howard, “Product composition andkinetics of lignite pyrolysis”, Ind. Eng. Chem. Process Des. Dev., 17:37-46, 1978.
[4] I.H. Rinard, B.W. Benjamin, “Great plains ASPEN model development:gasifier model. Literature Review and Model Specification”, U.S.Department of Energy, Morgantown, WV, Final Topical ReportDOE/MC/19163-1782, 1985.
[5] C.-Y. Wen, T.-Z. Chaung, “Entrainment coal gasification modeling”, Ind.Eng. Chem. Process Des. Dev., 18: 684-695, 1979.
[6] S.-S. Xu (许世森), D.-L. Zhang (张东亮), Y.-Q. Ren (任永强), “Large-scale
coal gasification technology (大规模煤气化技术)”, Beijing: Chemical
Industry Press, 2006.
[7] K.-F. Cen (岑可法), M.-J. Ni (倪明江), Z.-Y. Luo (骆仲泱), J.-H. Yan (严建
华), Y. Chi (池涌), M.-X. Fang (方梦祥), X.-T. Li (李绚天), L.-M. Cheng (程
乐鸣), “Theory, design and operation of circulating fluidized bed boilers (
循环流化床锅炉理论设计与运行)”, Beijing: Chinese Electric Power Press,
1998.
[8] B.W. Benjamin, “Great plains ASPEN model development: gasifiermodel”, Report submitted to U.S. Department of Energy, ContractDOE/MC/19163-1787, 1985.
[9] T. Krishnudu, B. Madhusudhan, S.N. Reddy, V.S.R. Sastry, K.S. Rao, R.Vaidyeswaran, “Studies in a moving bed pressure gasifier: prediction ofreaction zones and temperature profile”, Ind. Eng. Chem. Res., 28:438-444, 1989.
